1. Field of the Invention
The present invention is related to a production method for lot management. Specifically, the present invention relates to a lot management production method for a production line in which there is provided at least one apparatus of each variety of processing apparatuses for each variety of product, and a product-carrying container (for example, a wafer cassette) thereof. In such line, one lot of a designated number of pieces of a same variety/product (for example, a semiconductor wafer) loaded/carried in a carrying container constitutes a minimum unit for consideration. Also, the present invention also relates to a production method and a carrying container thereof, in which production is carried out in large volume of large variety and a whole set of processing steps is performed against each lot according to a sequence corresponding to the variety/product of the lot.
2. Related Art
A wafer process is essential to a semiconductor apparatus manufacturing process and requires a large variety of production processes such as oxidation, resist film coating, exposure, development, etching, diffusion, CVD (Chemical Vapor Deposition), PVD (Physical Vapor Deposition), etc. Such processes are performed on a production line having provided for each variety of product, one or a plurality of apparatuses corresponding to different processing steps.
In addition, a plurality of semiconductor wafers constituting semiconductor apparatuses of different varieties flow through the line on a lot basis. That is, a semiconductor wafer or piece in each lot is transported from a production line to an assembly line after passing through a set of processes according to a sequence determined for a corresponding variety of product to which the piece pertains. A lot is constituted by a predetermined number of semiconductor wafers, for example 24 pieces (alternatively, 25, 50, 100, etc.), and it has been usual practice to perform a whole set of processing steps for each variety, upon loading the number of pieces of semiconductor wafers constituting a lot of, for example, 24 pieces, on a wafer cassette. FIG. 5 schematically shows a lot flow of a conventional wafer processing production line.
In other words, in the conventional related art, each variety of products (herein after referred to simply as “product”) passes through a process that is independent from processes of other varieties. For example, a lot A of product “a” in FIG. 5 passes through a process of: Sheet-fed apparatus 1→ Batch apparatus 1→ Sheet-fed apparatus 101. Such sequence of processes is independent from other lots (lot B, C, etc. in the figure) of other products (“b”, “c”, etc.). Likewise, the same applies to the other lots B, C, etc. of other products “b”, “c”, etc. In addition, although a wafer processing production line is conventionally designed for a production of small variety in large volume, there have been attempts to adapt the line for large volume of a large variety. However, even though such attempts have been made for making it possible to realize a production line under large volume of large variety, the efforts have not been done with regard to large volume of small variety, but in order to keep and increase productivity.
However, as integration and downsizing of the semiconductor apparatuses increased, a larger variety of electronic circuits are included in a semiconductor apparatus and assembled in various kinds of electronic appliances. As a result, great progress has been achieved towards improvement of reliability of the electronic appliances, as well as their downsizing, price reduction, improvement of functions, etc., thus the semiconductor apparatuses have expanded their field of application. Consequently, there is an increased request for reduction of lead-time between order and delivery of products.
In other words, there are a considerable number of semiconductor apparatuses in a production line for wafer processing, as it has been described above (FIG. 5 is a schematic representation of a lot processing only for general description purposes, so that an actual line would have tens or dozens of processes). As there may be an assembly process following the wafer process, it is not rare to have a time required from start of production until delivery (a lead-time or lap-time) to be around 1 to 3 months. As a result, the order-issuing side issues an order based on this premise, and planning is conventionally made for manufacturing a product set (a radio, a television image receiver or the like) including the semiconductor apparatus as a component or part. Also, an ordered volume for a semiconductor apparatus of a same type used to be large. For example, it no rarely reached tens of thousands or even millions of pieces per month.
However, as the application for semiconductor apparatuses expanded, the semiconductor apparatuses started to be used also in products of relatively short lifetime as well as products of relatively small volume of production. For example, there is a trend for increasing a sort of order in which it is requested to deliver, for example, 2000 pieces of a designated variety of semiconductor apparatus within a period of 1 month. As a result, the semiconductor manufacturer has to cope with the burden of responding to such kind of request.
In view of such situation, semiconductor manufacturers currently make a forecast for prospective orders and produce in large quantity and keep a large intermediate inventory of a variety of semi-processed products, i.e., unfinished products still in process. Then, the remaining processes are performed accordingly whenever there is an actual order, thus allowing delivery as ordered. According to such procedure, it is supposed that the period of time required from order to delivery is reduced by the amount of time of the processes already performed upon the order forecast with the semi-processed products.
Especially for processes that have larger influence on the production lead-time (or lap-time), there is a strong tendency of producing semi-processed semiconductor wafers in large quantity, thus preparing an intermediate inventory that keeps standing by for prospective orders.
However, keeping large volume of intermediate inventory is not a recommended strategy since, as far as business management is concerned, it is unthrifty and constitutes as negative factor for the management. In addition, it is not rare a case in which the forecasted order has not actually been issued. In such a case, the standing by inventory of semi-processed semiconductors is wasted, causing considerable loss.
Also, once a semiconductor wafer enters an intermediate inventory, it may constitute a factor of oversetting the entire production process, as it is usual practice to perform the production process by giving priority to products having order information of higher accuracy. As a result, such intermediate inventory may give raise to considerable oscillation in production lead-time.
Moreover, if an actual ordered quantity is considerably smaller than lot that used to be of, for example, 24 pieces, the excessive pieces of the lot are turned to intermediate inventory, thus causing increase of the inventory.
As a countermeasure, it is possible to consider reducing the size of the lot of the semiconductor wafer to a fraction of the original lot, such as 6 pieces, for example, while keeping the basic rule of producing the semiconductor wafer by lot throughout all processing steps. By such procedure, as far as small-volume orders are concerned, it is expected that the lead-time required until delivery can be considerably reduced.
As an example, FIG. 6 shows a difference in lap-time (lead-time) according to difference in lot size, by means of a bar graph. Also, a broken line in the figure shows a number of processing apparatuses required according to the size of the lot. Still in FIG. 6, DRY represents dry etching, DIFF represents diffusion, CVD represents Chemical Vapor Deposition, PR represents Photo Resist process, II represents Ion Implantation process, PVD represents Physical Vapor Deposition, CMP represents Chemical Mechanical Polish, MES represents lot flow within the line. As it can be verified in the bar graph of FIG. 6, while in a current situation (current lot: in the example presented, 24 S/L means: 1 lot having 24 wafers; in addition, S means Slice, L means lot and S/L indicates a number of pieces (slices) of semiconductor wafers that constitute one lot) the product having a lap-time of 30 days, the lap-time is 15 days for 12 S/L, 10.2 days for 3 S/L and 9.2 days for 1 S/L, thus the lap-time is reduced proportionally to reduction of the lot size. Accordingly, it can be considered a procedure of reducing lap time by reducing the number of slices of semiconductor wafers in one lot.
Such procedure requires, however, increasing a number of processing apparatuses for keeping the same productivity, as shown in the broken line shown in FIG. 6. Specifically, while the current situation requires 88 machines, the case of 12 S/L requires 153 machines, 6 S/L requires 232 machines and 3 S/L requires 600 machines. This results in reduction of productivity of the production line. In other words, as efficiency of production is reduced as compared to an order for large volume of small variety, such procedure of reducing the size of the lot is not recommended as it hampers efficient utilization of the high productivity that could be attained by the production line.
Although there to be increasing trend towards ordering small volume of large variety, as far as management of production is concerned, there are still orders for large volume of small variety, which cannot be disregarded. Nevertheless, products having such ordering characteristic are products having lots of competitors and, as a result, their unit price is not high, making it unprofitable unless production is carried out with high efficiency. Accordingly, as far as feasibility of the production process is concerned, attempts at promoting productivity of small volume of large variety upon compromising the productivity of the large volume of small varieties may be prohibitive.
In addition, as a way of reducing lead-time, it is possible to consider reducing the lead-time as compared to the current situation, for example shown in FIG. 7A, by performing continuous processing of the lot, as shown in FIG. 7B. FIG. 7A shows a timeline required for a vacuum processing apparatus processing a current lot of, for example, 24 pieces, while FIG. 7B shows a case of reduction of lot to 12 pieces of semiconductor wafers, along with adding a load lock chamber to an existing load lock chamber of the processing apparatus above, thus making the processing chamber related to the apparatus operate under full workload. In other words, the processing shown in FIG. 7B is a case in which there is no spare time in the process performed in the chamber.
Specifically, depressurization is done at the load lock chamber, then the lot is conveyed (or transported) and, upon starting a designated process in the processing chamber, the next lot is depressurized, conveyed and, upon finishing the process, the next lot enters the processing chamber and processed, so that the processing chamber is fully loaded, with no spare time. By doing so, time is achieved in relation to the current situation in FIG. 7A, as it is clearly shown in FIG. 7B.
However, such procedure is not recommended because it requires increasing a number of load lock chambers for the existing processing apparatus, thus increasing the amount of investment required in equipment or machinery of the production line. This is especially true for processing apparatuses requiring depressurization, in which considerable new investment in equipment will be required, pushing the profit and loss break-even point upwards, thus constituting a negative factor for the management performance of the production line.